Conventionally, a shift position switch controller performs a switch control of an automatic transmission for a switching between a plurality of shift positions, according to an absolute rotation position of an output shaft that is rotated/driven by an electric motor via a rotation transmission mechanism.
A patent document, Japanese Patent No. 4385768 (patent document 1), for example, discloses such a shift position switch controller, in which a rotation position of the electric motor is controlled toward a target rotation position based on a detected position that is detected by an encoder. In such control of the rotation position toward the target rotation position, the target rotation position is set based on (i) an absolute rotation position of the output shaft detected by an output shaft sensor and (ii) the rotation position of the electric motor detected by the encoder. In such manner, the deterioration of accuracy of the shift position switch control due to a play/backlash of the rotation transmission mechanism is reduced.
However, the absolute rotation position of the output shaft detected by the output shaft sensor in the patent document 1 is prone to include an error that is caused by an installation position shift of the output shaft sensor, a variation of temperature characteristics of the sensor element, and the like. Thus, the rotation position of the electric motor controlled toward the target rotation position based on such an error-prone absolute rotation position is also prone to include an error. As a result, the rotation position of the electric motor may overshoot/undershoot the target rotation position, leading to the deterioration of accuracy of the shift position switch control.
Addressing such a problem, by (a) learning a reference rotation position in a certain reference shift position among a plurality of shift positions of the automatic transmission and (b) detecting a rotation position of the electric motor based on such a reference rotation position, which serves as a rotation start position, the rotation position of the electric motor minimizes the above-described error in the absolute rotation position of the output shaft. However, until an initialization of the learn position of the reference rotation position in the certain reference shift position, or until an input of a post-switch shift position in a shift position switch instruction after the initialization of the learn position being set as the reference shift position, the reference rotation position cannot be learned. As a result, based on a pre-learn reference rotation position, which is a rotation start position before accurate learning of the reference rotation position, the rotation position of the electric motor still either overshoots or undershoots in a rotation control toward the target rotation position thereby highly probably causing deteriorated accuracy in the shift position switch control. More specifically, the target rotation position of the shift position switch controller in the patent document 1 is under an influence of the absolute rotation position of the output shaft, which increases the degree of deterioration of accuracy of the shift position switch control.